Production of chemically functionalized nano graphene materials

ABSTRACT

Provided in this invention is a process for producing chemically functionalized nano graphene materials, known as nano graphene platelets (NGPs), graphene nano sheets, or graphene nano ribbons. Subsequently, a polymer can be grafted to a functional group of the resulting functionalized NGPs. In one preferred embodiment, the process comprises (A) dispersing a pristine graphite material and an azide or bi-radical compound in a liquid medium comprising to form a suspension; and (B) subjecting the suspension to direct ultrasonication with to ultrasonic waves of a desired intensity or power level for a length of time sufficient to produce nano graphene platelets and to enable a chemical reaction to occur between the nano graphene platelets and the azide or bi-radical compound to produce the functionalized nano graphene material. Concurrent production and functionalization of NGPs directly from pristine graphitic materials can be achieved in one step and in the same reactor.

The present invention is a result of a research and development projectsponsored by the US National Science Foundation Small BusinessTechnology Transfer (STTR) Program.

FIELD OF THE INVENTION

The present invention relates generally to the field of nano grapheneplatelets (NGPs), also known as graphene nano sheets or graphene nanoribbons. This invention provides methods and processes for chemicallyfunctionalizing NGPs, including both pristine NGPs and their oxidizedversions (graphite oxide nano platelets). A primary goal of producingchemically functionalized NGPs is improved solubility of NGPs in aliquid medium, improved dispersibility of NGPs in a matrix material, orenhanced interfacial bonding between NGPs and a matrix material in acomposite.

BACKGROUND OF THE INVENTION

Nanocomposites containing a nano-scaled filler possess unique featuresand functions unavailable in conventional fiber-reinforced polymers. Onemajor filler development in the past two decades is the carbon nanotube(CNT), which has a broad range of nanotechnology applications. However,attempts to produce CNT in large quantities have been fraught withoverwhelming challenges due to poor yield and costly fabrication andpurification processes. Additionally, even the moderately pricedmulti-walled CNTs remain too expensive to be used in high volume polymercomposite and other functional applications. Further, for manyapplications, processing of nanocomposites with high CNT concentrationshas been difficult due to the high melt viscosity.

Instead of trying to develop lower-cost processes for CNTs, theapplicants have sought to develop an alternative nanoscale carbonmaterial with comparable properties that can be produced much morecost-effectively and in larger quantities. This development work led tothe discovery of processes and compositions for a new class of nanomaterial now commonly referred to as nano graphene platelets (NGPs),graphene nano sheets, or graphene nano ribbons [e.g., B. Z. Jang and W.C. Huang, “Nano-scaled grapheme plates,” U.S. Pat. No. 7,071,258, Jul.4, 2006].

An NGP is a platelet, sheet, or ribbon composed of one or multiplelayers of graphene plane, with a thickness as small as 0.34 nm (one atomthick). A single-layer graphene is composed of carbon atoms forming a2-D hexagonal lattice through strong in-plane covalent bonds. In amulti-layer NGP, several graphene planes are weakly bonded togetherthrough van der Waals forces in the thickness-direction. Conceptually,an NGP may be viewed as a flattened sheet of a carbon nano-tube (CNT),with a single-layer graphene corresponding to a single-wall CNT and amulti-layer graphene corresponding to a multi-wall CNT. However, thisvery difference in geometry also makes electronic structure and relatedphysical and chemical properties very different between NGP and CNT. Itis now commonly recognized in the art of nanotechnology that NGP and CNTare two different and distinct classes of materials.

For more than six decades, scientists have presumed that a single-layergraphene sheet (one carbon atom thick) could not exist in its free statebased on the reasoning that its planar structure would bethermodynamically unstable. Surprisingly, several groups worldwide(including the applicants) have succeeded in obtaining isolated graphenesheets [e.g., B. Z. Jang, et al, U.S. Pat. No. 7,071,258 (patentapplication was submitted in October 2002); and K. S, Novoselov, et al.“Electric field effect in atomically thin carbon films,” Science 306,666-669 (2004)].

NGPs are predicted to have a range of unusual physical, chemical, andmechanical properties and several unique properties have been observed.For instance, single-layer graphene was found to exhibit the highestintrinsic strength and highest thermal conductivity of all existingmaterials [C. Lee, et al., “Measurement of the Elastic Properties andIntrinsic Strength of Monolayer Graphene,” Science, 321 (July 2008)385-388; A. Balandin, et al. “Superior Thermal Conductivity ofSingle-Layer Graphene,” Nano Lett., 8 (3) (2008) 902-907]. Single-sheetNGPs possess twice the specific surface areas compared withsingle-walled CNTs. In addition to single-layer graphene, multiple-layergraphene platelets also exhibit unique and useful behaviors.Single-layer and multiple-layer graphene are herein collectivelyreferred to as NGPs. Graphene platelets may be oxidized to variousextents during their preparation, resulting in graphite oxide (GO)platelets. In the present context, NGPs refer to both “pristinegraphene” containing no oxygen and “GO nano platelets” of various oxygencontents. It is helpful to herein describe how NGPs are produced.

The processes that have been used to prepare NGPs were recently reviewedby the applicants [Bor Z. Jang and A Zhamu, “Processing of Nano GraphenePlatelets (NGPs) and NGP Nanocomposites: A Review,” J. Materials Sci. 43(2008) 5092-5101]. As illustrated in FIG. 1, the most commonly usedprocess entails treating a natural graphite powder (referred to asProduct (A) in FIG. 1) with an intercalant and an oxidant (e.g.,concentrated sulfuric acid and nitric acid, respectively) to obtain agraphite intercalation compound (GIC) or, actually, graphite oxide (GO)(referred to as Product (B) in FIG. 1). Prior to intercalation oroxidation, graphite has an inter-graphene plane spacing of approximately0.335 nm (L_(d)=½ d₀₀₂=0.335 nm or 3.35 Å, based on X-ray diffractiondata readily available in open literature). There is a misconception inthe scientific community that van der Waals forces are weak forces,which needs some qualifications. It is well-known that van der Waalsforces are short range forces, but can be extremely strong in magnitudeif the separation between two objects (e.g., two atoms or molecules) isvery small, say <0.4 nm. However, the magnitude of van der Waals forcesdrops precipitously when the separation increases just slightly. Sincethe inter-graphene plane distance in un-intercalated and un-oxidizedgraphite crystal is small (<0.34 nm), the inter-graphene bonds (van derWaals forces) are actually very strong.

With an intercalation or oxidation treatment, the inter-graphene spacingis increased to a value typically greater than 0.55-0.65 nm. This is thefirst expansion stage experienced by the graphite material. The van derWaals forces are now significantly weakened due to the increasedspacing. It is important to note that, in most cases, some of thegraphene layers in a GIC are intercalated (with inter-graphene spacingincreased to 0.55-0.65 nm and van der Waals forces weakened), but otherlayers could remain un-intercalated or incompletely intercalated (withinter-graphene spacing remaining approximately 0.34 nm and van der Waalsforces staying strong).

In the conventional processes, the obtained GIC or GO, dispersed in theintercalant solution, will need to be rinsed for several cycles and thendried to obtain GIC or GO powders. These dried powders, commonlyreferred to as expandable graphite, are then subjected to furtherexpansion or second expansion (often referred to as exfoliation)typically using a thermal shock exposure approach (at a temperature from650° C. to 1,100° C.). The acid molecules residing in the inter-graphenespacing are decomposed at such a high temperature, generating volatilegas molecules that could push apart graphene planes. The inter-flakedistance between two loosely connected flakes or platelets is nowincreased to the range of typically >20 nm to several μm (hence, veryweak van der Waals forces).

Unfortunately, typically a significant portion of the gaseous moleculesescape without contributing to exfoliation of graphite flakes. Further,those un-intercalated and incompletely intercalated graphite layersremain intact (still having an inter-graphene spacing of approximately<0.34 nm). Additionally, many of the exfoliated flakes re-stack togetherby re-forming van der Waals forces if they could not be properlyseparated in time. These effects during this exfoliation step led to theformation of exfoliated graphite (referred to as Product (C) in FIG. 1),which is commonly referred to as “graphite worm” in the industry.

The exfoliated graphite or graphite worm is characterized by havingnetworks of interconnected (un-separated) flakes which are typically >50nm thick (often >100 nm thick). These individual flakes are eachcomposed of hundreds of layers with inter-layer spacing of approximately0.34 nm (not 0.6 nm), as evidenced by the X-ray diffraction data readilyavailable in the open literature. In other words, these flakes, ifseparated, are individual graphite particles, rather than graphiteintercalation compound (GIC) particles. This thermal shock procedure canproduce some separated graphite flakes or graphene sheets, but normallythe majority of graphite flakes remain interconnected. Again, theinter-flake distance between two loosely connected flakes or plateletsis now increased to from 20 nm to several μm and, hence, the ven derWaals forces that hold them together are now very weak, enabling easyseparation by mechanical shearing or ultrasonication.

Typically, the exfoliated graphite or graphite worm is then subjected toa sheet or flake separation treatment using air milling, mechanicalshearing, or ultrasonication in a liquid (e.g., water). Hence, aconventional process basically entails three distinct procedures: firstexpansion (oxidation or intercalation), further expansion (so called“exfoliation”), and separation. The resulting NGPs are graphene oxide(GO), rather than pristine graphene.

It is important to note that the separation treatment (e.g. usingultrasonication or shearing) is to separate those thick flakes from oneanother (breaking up the graphite worm or sever those weakinterconnections), and it is not intended for further peeling offindividual graphene planes. In the prior art, a person of ordinary skillwould believe that ultrasonication is incapable of peeling offnon-intercalated/un-oxidized graphene layers. In other words, in theconventional processes, the post-exfoliation ultrasonication procedurewas meant to break up graphite worms (i.e., to separate those alreadylargely expanded/exfoliated flakes that are only loosely connected).Specifically, it is important to further emphasize the fact that, in theprior art processes, ultrasonification is used after intercalation andoxidation of graphite (i.e., after first expansion) and most typicallyafter thermal shock exposure of the resulting GIC or GO (i.e., aftersecond expansion or exfoliation) to aid in breaking up those graphiteworms. There are already much larger spacings between flakes afterintercalation and/or after exfoliation (hence, making it possible toeasily separate flakes by ultrasonic waves). This ultrasonication wasnot perceived to be capable of separating thoseun-intercalated/un-oxidized layers where the inter-graphene spacingremains <0.34 nm and the van der Waals forces remain strong.

The applicant's research group was the very first in the world tosurprisingly observe that, under proper conditions (e.g., with theassistance of a surfactant), ultrasonication can be used to produceultra-thin graphene directly from graphite, without having to go throughchemical intercalation or oxidation. This invention was reported in apatent application [A. Zhamu, J. Shi, J. Guo, and Bor Z. Jong, “Methodof Producing Exfoliated Graphite, Flexible Graphite, and Nano GraphenePlates,” Pending U.S. patent Ser. No. 11/800,728 (May 8, 2007)].Schematically shown in FIG. 2 are the essential procedures used toproduce single-layer or few-layer graphene using this directultrasonication process. This innovative process involves simplydispersing graphite powder particles in a liquid medium (e.g., water,alcohol, or acetone) containing a dispersing agent or surfactant toobtain a suspension. The suspension is then subjected to anultrasonication treatment, typically at a temperature between 0° C. and100° C. for 10-120 minutes. No chemical intercalation or oxidation isrequired. The graphite material has never been exposed to any obnoxiouschemical. This process combines expansion, exfoliation, and separationinto one step. Hence, this simple yet elegant method obviates the needto expose graphite to a high-temperature, or chemical oxidizingenvironment. The resulting NGPs are essentially pristine graphene.

In order for NGPs to be an effective nano-filler or reinforcement in apolymer matrix, the surface of NGPs (either pristine graphene orgraphene oxide) must be properly functionalized for enhanced dispersionof NGPs in the matrix and improved compatibility or interfacial bondingbetween NGPs and the matrix polymer. Proper dispersion of NGPs in amatrix would be a prerequisite to achieving good electrical, thermal,and mechanical properties of the resulting composite materials. Hence,the objectives of our recent research and development efforts inchemical functionalization of NGPs that led to the instant applicationwere:

-   -   (A) To develop the ability to manipulate the electrical        conductivity of individual graphene sheets and that of polymer        composites with the specific goals of (1) achieving a high        electrical conductivity at a low percolation ratio; (2)        establishing a guideline for designing and producing graphene        composites with conductivity values within the ranges suitable        for static charge dissipation, EMI/RFI shielding, electrostatic        spray painting, and fuel cell bipolar plates, respectively. A        percolation ratio is the threshold weight fraction or volume        fraction of conductive fillers at which the filler particles        form a network of electron-conducting paths in an otherwise        insulating matrix materials, such as a polymer.    -   (B) To identify proper functional groups that will prevent        graphene sheets from re-stacking upon one another during the        preparation of any device or composite (e.g., for supercapacitor        electrodes).    -   (C) To develop chemical functionalization approaches that allow        for mass production of functionalized NGPs from both pristine        graphene and graphene oxide.    -   (D) To explore the opportunities of combining or integrating NGP        production and chemical functionalization operations into just        one or two simple steps.        These objectives have been achieved and are partially summarized        in the instant application.

SUMMARY OF THE INVENTION

As a first preferred embodiment, the present invention provides a highlyinnovative, “combined production-functionalization process” formanufacturing a chemically functionalized nano graphene materialdirectly from a pristine graphite material. The process comprises (A)Dispersing the pristine graphite material and an azide or bi-radicalcompound in a liquid medium (comprising a solvent if the compound is ina solid state) to form a suspension; and (B) Subjecting the suspensionto direct ultrasonication with ultrasonic waves of a desired intensityfor a length of time sufficient to produce nano graphene platelets andto enable a chemical reaction to occur between the nano grapheneplatelets and the azide or bi-radical compound to produce thefunctionalized nano graphene material. Concurrent production andfunctionalization of NGPs directly from pristine graphitic materials areachieved.

This strikingly simple and elegant process essentially integrates theprocedures of expanding, exfoliating, separating, and functionalizinggraphene planes from a pristine graphite material into ONE single step.No pre-intercalation or oxidation of graphite is required or needed.This is one of the examples of the innovative approaches of the instantapplication wherein the production of NGPs and chemicalfunctionalization of NGPs are conducted essentially at the same time inthe same reactor.

In this process, the azide or bi-radical compound may be added to theliquid medium (containing the pristine graphite material) concurrentlyor sequentially after direct ultrasonication of the graphite material isallowed to proceed for a short period of time. The starting pristinegraphite material may be selected from the group consisting of naturalgraphite, artificial graphite, highly oriented pyrolytic graphite,carbon fiber, graphite fiber, carbon nano-fiber, graphitic nano-fiber,meso-carbon micro-bead, graphitized coke, and combinations thereof.

The nano graphene platelets produced by this process comprise primarilysingle-layer or few-layer graphene. In the process, the chemicalfunctionalization reaction may be controllably limited to occur at anedge or edges of the nano graphene platelets. Alternatively, thechemical reaction may be allowed to occur to an edge and at least oneprimary surface (graphene plane) of the nano graphene platelets.

In this process, the azide or bi-radical compound may be selected fromthe group consisting of 2-Azidoethanol, 3-Azidopropan-1-amine,4-(2-Azidoethoxy)-4-oxobutanoic acid,2-Azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R—)-oxycarbonyl nitrenes, where R=any one of the following groups

and combinations thereof.

The process may further comprise a step of grafting a polymer chain to afunctional group of the functionalized nano graphene material to producea polymer-grafted nano graphene material. This feature enables thedesign and production of specially grafted NGPs for use as a nano fillerfor a specific polymer matrix.

In another preferred embodiment of the present invention, the processcould begin with a pre-intercalated, oxidized, or halogenated graphitematerial. This combined production-functionalization process comprises:(A) Producing exfoliated graphite from the pre-intercalated, oxidized,or halogenated (e.g. fluorinated) graphite material; (B) Dispersing theexfoliated graphite and an azide or bi-radical compound in a liquidmedium comprising a solvent to form a suspension; and (C) Subjecting thesuspension to ultrasonication with ultrasonic waves of a desiredintensity for a length of time sufficient to produce nano grapheneplatelets and to enable a chemical reaction to occur between the nanographene platelets and the azide or bi-radical compound to produce thefunctionalized nano graphene material. The step (A) of producingexfoliated graphite may comprise exposing the pre-intercalated oroxidized graphite material to a temperature higher than 200° C. (moretypically higher than 850° C.), a chemical (reactive with theintercalant), ultrasonic waves, or a combination thereof.

Again, in this process, the graphite material is not limited to naturalgraphite. It may be selected from the group consisting of naturalgraphite, artificial graphite, highly oriented pyrolytic graphite,carbon fiber, graphite fiber, carbon nano-fiber, graphitic nano-fiber,meso-carbon micro-bead, graphitized coke, and combinations thereof.Again, the produced nano graphene platelets comprise single-layergraphene. The chemical reaction may be controlled to occur only to anedge or edges of the nano graphene platelets, or to an edge and at leastone primary surface of the nano graphene platelets. The process mayfurther comprise a step of grafting a polymer chain to a functionalgroup of the functionalized nano graphene material to produce apolymer-grafted nano graphene material.

Still another preferred embodiment of the present invention is acombined production-functionalization process for producing a chemicallyfunctionalized nano graphene material from a pre-intercalated, oxidized,or halogenated graphite material. The process comprises: (A) Dispersingthe pre-intercalated, oxidized, or halogenated graphite material and anazide or bi-radical compound in a liquid medium comprising a solvent toform a suspension; (B) Subjecting the suspension to ultrasonication withultrasonic waves of a desired intensity for a length of time sufficientto produce nano graphene platelets and to enable a chemical reaction tooccur between the nano graphene platelets and the azide or bi-radicalcompound to produce the functionalized nano graphene material.

In this process, the pre-intercalated, oxidized, or halogenated graphitedoes not have to go through exfoliation (to produce exfoliated graphiteworms) and flake separation (breakage of graphite worms). These twoprocedures are automatically accomplished concurrently with the chemicalfunctionalization procedure in the ultrasonication reactor vessel.Alternatively, the azide compound may be added to the liquid mediumsequentially after direct ultrasonication of the graphite material isallowed to proceed for a desired period of time. This is another exampleto illustrate the innovation that NGP production and chemicalfunctionalization are achieved concurrently and in the same reactor.This is also a highly innovative process that has never been disclosedor even slightly hinted in the prior art.

In another process, the NGPs are made on a separate basis. The processcomprises (A) mixing a starting nano graphene material (having edges andtwo primary surfaces), an azide or bi-radical compound, and an organicsolvent in a reactor, and (B) allowing a chemical reaction between thenano graphene material and the azide compound to proceed at atemperature for a length of time sufficient to produce thefunctionalized nano graphene material.

The starting nano graphene material may comprise pristine graphene,graphene oxide, graphene fluoride, graphene chloride, or a combinationthereof. Again, the chemical reaction may be prescribed to occur only toan edge or edges of the nano graphene material, or to an edge and atleast one primary surface of the nano graphene material. A wide range ofazide or bi-radical compounds may be used in the process. The processmay further comprise a step of grafting a polymer chain to a functionalgroup of the functionalized nano graphene material to produce apolymer-grafted nano graphene material.

In general, a combined production-functionalization process formanufacturing a chemically functionalized nano graphene material from agraphite material has been developed. This highly innovative processcomprises: (A) Dispersing a graphite material and a bi-functional ormulti-functional compound in a liquid medium to form a suspension; and(B) Subjecting the suspension to direct ultrasonication with ultrasonicwaves of a desired intensity for a length of time sufficient to producenano graphene platelets and to enable a chemical reaction to occurbetween the nano graphene platelets and the compound to produce thefunctionalized nano graphene material.

The bi-functional or multi-functional compounds have two, three, four,or more functional groups (e.g., —NH₂) at their two, three, four, ormore ends, respectively. At least one of the functional groups iscapable of reacting with an NGP at an edge or graphene plane.

The graphite material may be selected from a wide range of graphiticmaterials, including natural graphite, artificial graphite, highlyoriented pyrolytic graphite, carbon fiber, graphite fiber, carbonnano-fiber, graphitic nano-fiber, meso-carbon micro-bead, graphitizedcoke, pre-intercalated versions thereof, pre-oxidized versions thereof,pre-fluorinated versions thereof, chemically modified versions thereof,and combinations thereof. Oxidation, fluorination, and other chemicalmodifications (e.g., halogenation) of graphite are well-known in theart. The presently invented process is applicable to pristine versionsand various chemically modified versions of the above-listed graphiticmaterials.

The chemically functionalized nano graphene platelets produced with thisprocess typically comprise a significant portion of single-layergraphene. The chemical reaction can be controlled to occur only to anedge or edges of the nano graphene platelets or, alternatively, to anedge and at least one primary surface, graphene plane, of said nanographene platelets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Conventional, most commonly used chemical processes for producingoxidized NGPs or GO platelets.

FIG. 2 A surfactant-assisted direct ultrasonication method for theproduction of pristine graphene, as disclosed earlier by the instantapplicants.

FIG. 3 Schematic showing the reactions between pristine graphene andseveral azide compounds (as illustrative examples) to formfunctionalized NGPs.

FIG. 4 Electrical conductivity data for the thin films made frompristine NGPs (p-NGPs), GO, and CNTs after various periods of aminoazide reactions at 100° C.

FIG. 5 Electrical conductivity data for the thin films made frompristine NGPs (p-NGPs), GO, and CNTs after various periods of aminoazide reactions at 160° C.

FIG. 6 (a) chemical functionalization of NGPs with azide groups occursto the NGP edges first, which improves the solubility or dispersibilityof NGPs without significantly inducing changes to the properties of thegraphene plane; (b) As azide compound reactions proceed further,functionalization occurs to the graphene plane itself.

FIG. 7 Grafting of different polymers (a)-(c) or attaching Pt nanoparticles (d) to various functionalized NGPs, as means to verify thepresence of functional groups.

FIG. 8 Schematic of the three routes along which we can producechemically functionalized NGPs. The process can begin with dispersing apristine graphite material, a pre-intercalated or oxidized graphite (noexfoliation and flake separation), or an exfoliated graphite material(with or without flake separation) to obtain a suspension, followed byultrasonication step.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Successful functionalization reactions for single-walled (SWNT) andmulti-walled (MWNT) carbon nanotubes have been achieved by severalgroups worldwide. These approaches include defect functionalization,covalent functionalization of the sidewalls, non-covalent exohedralfunctionalization (e.g., formation of supramolecular adducts withsurfactants or polymers), and endohedral functionalization. A mostpopular approach involves the use of the nanotube-bound carboxylicacids, which are created using strong chemicals, such as concentratedsulfuric acid. The nanotube-bound carboxylic acids are the sites towhich a variety of functional groups for the solubilization of CNTs canbe attached. These solubilized CNTs allow for solution-basedcharacterizations and investigations.

Due to the similarity in chemical composition and structure betweencarbon nanotubes (CNTs) and nano graphene platelets (NGPs), one ofordinary skills might expect that all functionalization approaches forCNTs should work for functionalizing NGPs equally well. However, thehighly significant differences in geometry and morphology between CNTsand NGPs, as illustrated below, would suggest that the functionalizationapproaches that work for CNTs may not necessarily work for NGPs, andvice versa. Examples of these differences are:

-   -   (a) Geometrically, CNTs are quasi-one-dimensional entities (thin        tubes) while NGPs are quasi-two-dimensional (thin sheets);    -   (b) Although the basic molecular formulation of an NGP is        identical to that of a CNT (both being composed of all carbon        atoms arranged in a hexagon-type structure), the CNT molecule is        curved or rolled up to form a cylindrical shape, effectively        changing the electronic structure and related physical and        chemical properties;    -   (c) NGPs have two primary external surfaces that are flat while        CNTs have only one external surface that is convex (the interior        surface is not readily accessible by chemical species);    -   (d) Compared to CNTs, NGPs have more edges or more sites where        desirable or undesirable functional groups tend to come into        existence during the NGP producing process. These functional        groups make the characteristics of NGPs fundamentally different        from those of CNTs;    -   (e) In particular, most of the prior art processes are based on        chemical oxidation or intercalation of natural graphite,        producing NGPs typically in the state of graphene oxide (GO)        that already carries oxygen-containing groups, such as hydroxyl,        carbonyl, and/or carboxyl. However, in conventional        NGP-producing processes, the nature and amount of these groups        could not be well-controlled.    -   (f) In the non-covalent exohedral functionalization of a CNT        with a polymer, the entire polymer chain can wrap around a thin        tube like a helix. This would normally not be possible for NGPs        since the lateral dimensions of a chemically produced graphene        sheet are typically much greater than 0.3 μm, more typically        between 0.5 μm and 10 μm.

The results of our intensive and extensive research endeavors have beenmost surprising. In general, when applied to CNTs and NGPs, the sametype of chemical functionalization approach works to very differentextents, resulting in vastly different improvements in properties forthese two classes of carbon nano materials. In many cases, one approachworks for one of the two classes of nano materials, but not for theother class.

Further, our purposes of carrying out these research and developmentefforts were not primarily for trying to differentiate the effectivenessof various chemical approaches to functionalizing NGPs from that ofCNTs. Instead, we were more interested in finding ways of controllablyvarying the chemical and physical properties of graphene andgraphene-containing composites or devices. In particular, we recognizedthat the unique electrical and thermal properties of graphene were amongits most promising features for future applications. Understanding thesefeatures, and the ways to manipulate and control them, is therefore ofparamount importance for practical applications of various forms ofgraphene. In this regard, a basic understanding of the properties ofpristine graphene provides the necessary background for furtherdevelopments. In particular, chemical functionalization could be usedfor engineering the properties of graphene towards specificapplications.

In several earlier studies, attempts were made to employ some biradicalgroups, such as dichlorocarbene and nitrene, to modify the double bondsof CNTs [e.g., M. Holzinger, et al., J. Am. Chem. Soc. 125 (2003)8566-8580] and fullerenes [e.g., A. Yashiro, et al., Tetrahedron Lett.39 (1998) 9031-9034 and T. Nakahodo, et al., Angew. Chem., Int. Ed. 47(2008) 1298-1300]. Thus, our first task in the study of chemicalfunctionalization of NGPs was to determine if such carbene or nitrenechemistry could provide an approach for large-scale synthesis offunctionalized NGPs. We thought that this approach, if proven feasible,would be advantageous because of the high reactivity of azides and theiramenability to be cost-effectively produced in large quantities. Aspecific goal was to develop a simple or single-step process forlarge-scale production of soluble or dispersible NGPs with a highdensity of functional groups from both pristine nano graphene (p-NGP)and graphene oxide (GO). This process could provide facile, green, andcost-effective production of functionalized NGPs.

To evaluate the technical feasibility of the single-step methodology, aseries of functional bi-radical or azide compounds (f-azides) were usedto react with both single-layer and multi-layer NGPs and, for comparisonpurposes, both single-wall and multi-wall CNTs. Each f-azide moleculeutilized contains an azido group on one end and a functional group(e.g., —OH, —NH2, —COOH, or —Br) on another end, as schematically shownin FIG. 3. These were used as examples only, not intended for limitingthe scope of our invention, nevertheless. It was hoped that once theazido group anchors on a primary surface of an NGP, a second functionalgroup extends into the surrounding solvent to help solubilize the NGP.Hopefully, this second functional group is suitable for further chemicalmodification if so desired.

It is well-known in the art [e.g., M. Holzinger, et al., Carbon, 42(2004) 941-947] that an azido group may produce two types of reactiveintermediate upon thermolysis, i.e., singlet-state nitrenes (having twofilled p-orbitals) and triplet-state nitrenes (having one filledp-orbital with two p-orbitals containing unpaired electrons). Both typeswere known to be capable of attacking the surface of CNTs to impartaziridine rings by an electrophilic [2+1] cycloaddition and reactionbetween biradicals and the CNT surface's π-system, respectively. In thisR&D task, we sought to determine if an azido group could react with ourNGPs or somehow anchor on NGPs. We recognized that the procedures toproduce f-azides in large quantities from commercially availablereagents have been developed. If functional azides can react with NGPs,then functionalized NGPs (f-NGPs) can be cost-effectively produced inlarge quantities.

In the beginning, we envisioned a strategy for the preparation offunctionalized NGPs (f-NGPs), as illustrated in FIG. 3, wherein theazides act as an anchor that attaches a functional group to a surface(or both surfaces) of an NGP. These azide reagents can be easilysynthesized from NaN₃ and other readily available chemicals. Triazolinescould be formed by 1,3-dipolar cycloaddition reactions between organicazides and C═C bonds of graphene surfaces with a concomitant nitrogenloss occurring upon thermolysis [A. Yashiro, et al., Tetrahedron Lett.39 (1998) 9031-9034]. This thermolysis can be visually confirmed by thebubbles coming out of the reaction flask. In addition, thermalactivation of alkyl azide precursors should yield some azo andheterocyclic byproduct, causing the reaction solution to become darkbrown [P. N. D. Singh, et al., J. Am. Chem. Soc. 129 (2007)16263-16272].

With this approach in mind, we attempted to obtain four types of modelf-NGPs with different reagents of azides: hydroxyl-functionalized NGPs(NGP-OH), amino-functionalized NGPs (NGP-NH₂), carboxyl-functionalizedNGPs (NGP-COOH), and bromine-functionalized NGPs (NGP-Br). In ourstudies, functional groups were allowed to react with NGPs to obtain thef-NGPs by mixing pristine NGPs (p-NGPs) or graphene oxide (GO) andfunctional azides (f-azides) in N-methyl-2-pyrrolidone (NMP) at 160° C.for up to 5 hours. NMP can be readily recycled in the process and ismuch less expensive than 1,1,2,2-tetrachloroethane (TCE), which wascommonly used in functionalizing CNTs. The same reaction conditions wereemployed to obtain f-CNTs for comparison purposes. In a separate set ofsamples, both NGPs and CNTs were subjected to the same reactionconditions, but at a much lower temperature (100° C.) for several hours.

Before the reaction was initiated, nitrogen bubbling was allowed toproceed for several minutes to prevent the highly reactive intermediatesfrom reacting with oxygen. Ultrasonication was used to facilitatedispersion of pristine NGPs and CNTs in NMP. Upon completion ofnecessary reactions, the raw products were separated using directfiltration with filter paper for a large batch, or using precipitationin acetone followed by centrifugation for a small batch. Final f-NGPproducts were obtained after repeated washing with water or organicsolvent.

Six series of thin films were prepared from amino-functionalized f-NGPsand f-CNTs for the purpose of measuring the electrical conductivity ofthe functionalized p-NGPs (f-p-NGPs), functionalized GO nano platelets(f-GO), and f-CNTs as a function of reaction time and temperature. Eachfunctionalized material was re-dispersed in water to produce asuspension. Thin films from these suspensions were obtained by spincoating. Typical thickness of these thin films was in the range of 2-5μm. Shown in FIG. 4 and FIG. 5 are a summary of the electricalconductivity data of the films made from p-NGPs, GO, and CNTs aftervarious periods of amino azide reactions at 100° C. and 160° C.,respectively.

FIG. 4 indicates that, at a relatively low reaction temperature (100°C.), both amino-functionalized pristine NGPs and amino-functionalizedgraphene oxide began to exhibit a significant decrease in electricalconductivity after the reaction proceeded for >1.5 hours. In contrast,the electrical conductivity of CNTs remained essentially unchanged evenafter 5 hours of azide reactions. In order to help us understand whathas happened, if any, to these carbon nano materials as a function ofthe reaction time, we investigated the infrared spectra of thesesamples. The results were very surprising. Both pristine NGPs and GOsamples, prior to azide reaction, exhibited no signals corresponding toamino group (—NH₂). However, after only 30 minutes of reaction, bothsamples began to exhibit the presence of amino groups and the amounts ofamino group increased with increasing reaction time. This was not thecase for CNT samples. After 5 hours of reaction at 160° C., there was nosign of —NH₂ group being attached to CNTs. This was consistent with thenotion that no electrical conductivity change was observed over areaction period of 5 hours.

The next logical question to ask was why both p-NGPs and GO began tocapture the azide group at the early stage of azide reaction, yet didnot experience a reduction in electrical conductivity until the reactiontimes reached 1.5 or 2.0 hours and beyond. Not wishing to be bound byany theory, we speculated that one of the end groups of the azidecompound could easily attach to the edges of both p-NGP and GO sheets(or molecules) where dangling bonds, other functional groups, orstructural defects were formed during the process of preparing bothp-NGPs and GO. There are large amounts of edge surfaces in nano grapheneplatelets or sheets, including both pristine NGPs or GO. By contrast,there is no edge surface for a pristine CNT; only very little amount ofend surface exists at the nanotube tip.

Presumably, the functional groups or defects at the edges of a nanographene sheet or platelet do not have an adverse effect on itselectrical conductivity since electrons move primarily through thegraphene plane, not along the edges. If or when functional groups ordefects are attached to the two primary surfaces (graphene planes) of agraphene sheet or platelet, the electron mobility would be significantlycurtailed since these defect sites would scatter electrons.

In this context, since GO platelets typically already had a certaindensity of defects or functional groups at the edges and on the grapheneplanes when they were produced, they exhibited a lower conductivitycompared with pristine NGPs prior to chemical functionalization. Whenazide groups were brought in contact with GO platelets, functionalgroups attacked both the edges and the primary surfaces (grapheneplanes), replacing pre-existing groups. These replacements did not leadto any significant reduction in conductivity; this reduction becameappreciable only after new defect sites were created by additionalreactions, possibly after a majority of pre-existing groups werereplaced.

For a pristine graphene sheet or platelet, it is reasonable to statethat the azide molecules began to attack the more vulnerable edges andwould likely cover a significant or a major portion of the availablesites at the edges before they attack the much stronger bonds inside agraphene plane. This implies that only after 1.5 hours of reaction thatthe azide molecule began to anchor one of its ends to the primarysurface of graphene, thereby inflicting a reduction in electricalconductivity. These observations further validate the assertion thatNGPs and CNTs are distinct classes of carbon nano materials and theirchemical and physical behaviors are very different.

The data shown in FIG. 5 and the FTIR spectra also indicate that thehigher temperature reactions between the azide compound and NGPs beganmuch sooner than those between the azide and CNTs, which did not occurfor 3 hours at 160° C. The former reactions began as soon as NGPs (bothp-NGPs and GO) were brought into contact with the azide compound, asevidenced by the infrared and Raman spectra. During the first hour, thechemical groups pre-existing at the graphene edges were rapidly replacedby the azide compound, but this substitution did not significantlyaffect the electrical conductivity of NGPs. Only after the chemicalgroups on the GO surfaces were substantially replaced by azide groups orwhen azide groups began to anchor onto the graphene plane of p-NGP,could we begin to observe a significant reduction in electricalconductivity.

In summary, as illustrated in FIG. 6( a), chemical functionalization ofNGPs with azide groups occur to the NGP edges first, which improves thesolubility or dispersibility of NGPs without significantly inducingchanges to the properties of the graphene plane. As azide compoundreactions proceed further, functionalization occurs to the grapheneplane itself (FIG. 6( b)). Such a sequence is not observed with thefunctionalization of CNTs.

The above discussions indicate that azide groups readily react with NGPsunder very mild conditions (e.g., at a temperature as low as 100° C.),but not CNTs. For NGPs, reactions typically began as soon as thereactants are mixed together and are typically completed within 1 to 2hours. For CNTs, reactions either did not occur at all (e.g., at 100°C.) or, if ever initiated (e.g. at 160° C. or greater), could not becompleted for 4-6 hours. The presently invented graphenefunctionalization process, being so simple and involving only mildreaction conditions, appears to be amenable to scaling up for massproduction. The advantages of the invented functionalization approachinclude: being a green process, involving only inexpensive or recyclablematerials, few reaction steps (essentially one step), and the functionalgroup density being controllable.

Thus, one of the preferred embodiments of the present invention is aprocess for producing chemically functionalized nano graphene materials.This process includes mixing a nano graphene material, an azide compound(including bi-radical compounds), and an organic solvent in a reactorand allowing a chemical reaction between the nano graphene material andthe azide compound to proceed at a temperature for a length of timesufficient to produce the desired functionalized NGPs. The starting nanographene material can be pristine graphene, graphene oxide, or acombination of both. The nano graphene material may be produced from anytype of graphitic material, not just natural graphite, which was used inall prior art processes (other than those used by the applicants). Thegraphitic material may be selected from the group consisting of naturalgraphite, artificial graphite, highly oriented pyrolytic graphite,carbon fiber, graphite fiber, carbon nano-fiber, graphitic nano-fiber,meso-carbon micro-bead, graphitized coke, and combinations thereof.

The azide compounds herein discussed may be selected from the groupconsisting of 2-Azidoethanol, 3-Azidopropan-1-amine,4-(2-Azidoethoxy)-4-oxobutanoic acid,2-Azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R—)-oxycarbonyl nitrenes, and combinations thereof, where R=any one ofthe following groups

Once the functional groups are covalently anchored to the NGPs, the nextlogical step is to determine if f-NGPs would exhibit improvedsolubility/dispersibility in a solvent or enhanced interfacial adhesionbetween NGPs and a polymer matrix in a composite. Further, it is oftechnological significance to determine if it is possible to directlygraft polymers from the NGP surfaces with the functional groups actingas initiating sites. A positive answer to this question would mean thatpolymer-grafted NGPs, which have a broad array of potentialapplications, can be produced cost-effectively.

To obtain answers to these questions, several chemical reactions wereperformed on the f-NGP, as shown in FIG. 7( a)-FIG. 7( d).Surface-initiated polymerization, generally termed the “grafting from”approach, has been adopted to covalently graft a range of polymer chainsfrom the surface of CNTs. To verify if hydroxyl groups were successfullyadded to NGPs, ring-opening polymerizations (ROP) of ε-caprolactone andcationic polymerization of glycidyl methacrylate (GMA) from NGP-OH werecarried out to produce NGP-g-PCL and NGP-g-PGMA, respectively (FIG. 7(a)), which were confirmed by spectroscopy data.

NGP-NH₂ was reacted with palmitoyl chloride to verify that amino groupswere indeed introduced onto NGPs (FIG. 7( b)). In order to furtherdemonstrate that bromine groups were successfully linked to NGPs to formNGP-Br, in situ atom transfer radical polymerization (ATRP) was carriedout with NGP-Br as a macro-initiator, and styrene, methyl methyacrylate(MMA), and 3-azido-2-hydroxypropyl methacrylate (GMAN3) as monomers toobtain NGP-g-PS, NGP-g-PMMA, and NGP-g-PGMAN3, respectively (FIG. 7(c)). Additionally, Pt/NGP nano-hybrids were prepared from NGP-COOH byreducing K₂PtCl₄ in an ethylene glycol-water solution that furtherconfirmed the presence of carboxyl groups (FIG. 7( d)).

Thus, another preferred embodiment of the present invention is a processfor producing chemically functionalized and polymer-grafted nanographene materials. This process includes mixing a nano graphenematerial, an azide compound, and an organic solvent in a reactor andallowing a chemical reaction between the nano graphene material and theazide compound to proceed at a temperature for a length of timesufficient to produce NGPs with a desired functional group attachedthereto. This step is followed by a chain grafting or polymerizing stepby which a polymer chain is attached to or reacted with the desiredfunctional group.

Up to this point of discussion, one may conclude that the proposednitrene chemistry-based, single-step production technology for preparingfunctional NGPs has several major advantages:

-   -   (1) Azides (or bi-radical compounds) can be synthesized in large        quantities under relatively mild conditions;    -   (2) The process is environmentally friendly since the decomposed        gas is nitrogen and the solvent can be recycled;    -   (3) The functionalization process does not induce severe damage        to NGPs;    -   (4) Almost no other functional group except the desired one is        anchored on the NGPs, making the f-NGPs structurally        well-defined materials;    -   (5) Various functional groups (for example, —OH, —NH2, —COOH,        —Br) can be introduced onto NGPs in merely one reaction;    -   (6) The reaction can be easily performed by thermolysis; and    -   (7) The approach is applicable to functionalization of both        pristine NGPs and oxidized NGPs (or GO).

The ability to functionalize the edges only, without inflicting damageto the bulk or in-plane structure of a graphene sheet, is a highlydesirable feature. This would enable good solubility of NGPs in asolvent for subsequent processing of NGPs or NGP composites withoutadversely altering their properties. This feature is not available tothe functionalization of CNTs. This feature would also enable improvedchemical compatibility or interfacial bonding between NGPs and a matrixpolymer without compromising the structure and properties of the NGPs,resulting in significantly improved composite properties.

In some of the graphene oxide-producing processes developed by us, acontrolled density of carboxylic acid groups is naturally attached tothe surfaces or edges of graphene oxide (GO) platelets when they aremade. Although carboxylic acid-laden GO platelets are soluble in waterand other highly polar solvents, such as alcohols, they are not solublein many other useful solvents. Further, upon removal of solvent, theresulting GO platelets tend to agglomerate through van der Waalsforce-induced re-stacking of GO nano sheets. Furthermore, GO plateletswith carboxylic acids do not necessarily provide the best interfacialbonding between NGPs and a desired polymer matrix. Other functionalgroups may be more effective in promoting interfacial bonding in aparticular composite material. The present invention provides aneffective approach to overcome the aforementioned issues.

In summary, the present invention provides a single-step process forchemically functionalizing the NGPs that were prepared independently andprior to the functionalization operation. These NGPs could be producedby any process schematically shown in FIG. 1 or FIG. 2.

Additionally, the applicants proceeded to carry out further research anddevelopment efforts to explore the opportunities of integrating orcombining the NGP production operation and NGP functionalizationoperation into one step, as opposed to sequentially producing NGPs andthen chemically functionalizing the NGPs on a separate basis.

As described earlier in the Background of the Invention section andspecifically illustrated in FIG. 2, the applicant's research group wasthe very first to discover the direct ultrasonication process capable ofmanufacturing ultra-thin graphene in large quantities directly frompristine graphite, without prior chemical intercalation or oxidation.This direct ultrasonication process involves simply dispersing pristinegraphite powder particles in a liquid medium (e.g., water, alcohol,acetone, or other solvent) containing a dispersing agent or surfactantto obtain a suspension. The suspension is then subjected to anultrasonication treatment, typically at a temperature between 0° C. and100° C. for 10-120 minutes. No prior chemical intercalation or oxidationis required. The pristine graphite material has never been exposed toany obnoxious chemical. This process combines expansion, exfoliation,and separation into one step, obviating the need to expose graphite to ahigh-temperature or chemical oxidizing environment.

This direct ultrasonication approach was herein extended to concurrentlyproduce NGPs and functionalize NGPs, as illustrated as Route 1 in FIG.8. This modified process entails replacing water with NMP and adding anazide compound and graphite powder into the liquid solvent in a reactor.The resulting suspension is then subjected to ultrasonication, typicallyfor a period of time in the range of 20 to 120 minutes. We were mostsurprised to observe that the product was ultra-thin nano grapheneplatelets (with lots of single-layer graphene) that have been chemicallyfunctionalized. In other words, we succeeded in integrating theoperations of graphite expansion, exfoliation, separation, and chemicalfunctionalization into ONE single step. This is in stark contrast to theconventional NGP production processes that require so many steps andundesirable chemicals, as illustrated in FIG. 1. This strikingly simpleprocess of manufacturing functionalized NGPs directly from a graphitematerial and in large quantities is truly a breakthrough in the art ofnano graphene.

Hence, another preferred embodiment of the present invention is acombined production-functionalization process for manufacturing achemically functionalized nano graphene material directly from apristine (non-intercalated and non-oxidized) graphite material. Thisintegrated process comprises (A) Dispersing the graphite material and anazide compound in a liquid medium comprising a solvent to form asuspension in a reactor; and (B) Subjecting the suspension to directultrasonication with ultrasonic waves of a desired intensity for alength of time sufficient to produce nano graphene platelets and toenable a chemical reaction to occur between the nano graphene plateletsand the azide compound to produce the desired functionalized nanographene material. This graphite material may be selected from the groupconsisting of natural graphite, artificial graphite, highly orientedpyrolytic graphite, carbon fiber, graphite fiber, carbon nano-fiber,graphitic nano-fiber, meso-carbon micro-bead, graphitized coke, andcombinations thereof.

In this direct ultrasonication process, the azide compound may be addedto the liquid medium before the ultrasonic power is turned on.Alternatively, the azide compound may be added sequentially after thegraphite material is ultrasonicated for some period of time. Bothprocedure sequences are very effective in generating functionalizedNGPs.

Further alternatively, functionalized NGPs may be manufactured fromintercalated or oxidized graphite, as illustrated as Route 2 and Route 3in FIG. 8. In Route 2, the process begins with preparation of oxidizedgraphite or graphite oxide powder. The oxidized graphite or graphiteoxide was not subjected to exfoliation and separation treatments.Instead, the oxidized graphite powder and an azide compound were addedto a solvent to produce a graphite oxide-azide suspension in a reactor.This suspension is then subjected to ultrasonication, which assists inexfoliation, separation, and functionalization all at the same time toproduce functionalized NGPs. In other words, the exfoliation andseparation operations of the oxidized graphite were integrated with thechemical functionalization operation into one step, which is carried outinside the same reactor.

In Route 3 of FIG. 8, the graphite material is intercalated, oxidized,or halogenated to produce graphite intercalation compound (GIC),graphite oxide, or halogenated graphite. The GIC, graphite oxide, orhalogenated graphite is then exposed to a thermal shock (e.g., at atemperature >300°-1,050° C. for 30-60 minutes) to produce exfoliatedgraphite (graphite worms). Without subjecting to a prior separationtreatment (e.g., using air jet mill, high-shear mixer, orultrasonicator), the exfoliated graphite is mixed with an azide compoundin a solvent contained in a reactor. The resulting suspension is thensubjected to ultrasonication, which not only break up graphite worms toform separated NGPs, but also functionalize the NGPs substantially atthe same time in the same reactor.

The three processes depicted in FIG. 8 are highly innovative and havenot been taught implicitly or explicitly in the prior art. Hence,another preferred embodiment of the present invention is a combinedproduction-functionalization process for manufacturing a chemicallyfunctionalized nano graphene material directly from a non-intercalatedand non-oxidized graphite material (Route 1). This process comprises (A)dispersing the graphite material and an azide compound in a liquidmedium comprising a solvent to form a suspension; and (B) subjecting thesuspension to direct ultrasonication with ultrasonic waves of a desiredintensity for a length of time sufficient to produce nano grapheneplatelets and to enable a chemical reaction to occur between the nanographene platelets and the azide compound to produce the functionalizednano graphene material.

Still another preferred embodiment of the present invention is acombined production-functionalization process for manufacturing achemically functionalized nano graphene material directly from anintercalated, oxidized, or halogenated graphite material (Route 3). Thisprocess comprises (A) producing exfoliated graphite from theintercalated or oxidized graphite material; (B) dispersing theexfoliated graphite and an azide compound in a liquid medium comprisinga solvent to form a suspension; and (C) subjecting the suspension toultrasonication with ultrasonic waves of a desired intensity for alength of time sufficient to produce nano graphene platelets and toenable a chemical reaction to occur between the nano graphene plateletsand the azide compound to produce the functionalized nano graphenematerial.

A further preferred embodiment of the present invention is a combinedproduction-functionalization process for manufacturing a chemicallyfunctionalized nano graphene material directly from an intercalated,oxidized, or halogenated (e.g., fluorinated) graphite material (Route2). The process comprises (A) dispersing said intercalated, oxidized, orfluorinated graphite material and an azide compound in a liquid mediumcomprising a solvent to form a suspension; and (B) subjecting thesuspension to ultrasonication with ultrasonic waves of a desiredintensity for a length of time sufficient to produce nano grapheneplatelets and to enable a chemical reaction to occur between the nanographene platelets and the azide compound to produce the functionalizednano graphene material.

In general, a combined production-functionalization process formanufacturing a chemically functionalized nano graphene material from agraphite material has been developed. This highly innovative processcomprises: (A) Dispersing a graphite material and a bi-functional ormulti-functional compound in a liquid medium to form a suspension; and(B) Subjecting the suspension to direct ultrasonication with ultrasonicwaves of a desired intensity or power level for a length of timesufficient to produce nano graphene platelets and to enable a chemicalreaction to occur between the nano graphene platelets and the compoundto produce the functionalized nano graphene material.

The bi-functional or multi-functional compounds have two, three, four,or more functional groups (e.g., —NH₂) at their two, three, four, ormore ends, respectively. At least one of the functional groups iscapable of reacting with an NGP at an edge or graphene plane. Azidecompounds are among many available di-functional and multi-functionalcompounds suitable for use in this highly versatile process. It appearsthat high-power ultrasonication is capable of activating the edges orgraphene planes of NGPs and enabling many functionalization reactions toreadily initiate and proceed. The graphite material may be selected froma wide range of graphitic materials, including natural graphite,artificial graphite, highly oriented pyrolytic graphite, carbon fiber,graphite fiber, carbon nano-fiber, graphitic nano-fiber, meso-carbonmicro-bead, graphitized coke, pre-intercalated versions thereof,pre-oxidized versions thereof, pre-fluorinated versions thereof,chemically modified versions thereof, and combinations thereof.Oxidation, fluorination, and other chemical modifications (e.g.,bromination) of graphite are well-known in the art. For instance,fluorinated graphite can be prepared according to several processescited in R. Yazami et al [“Subfluorinated graphite fluorides aselectrode materials,” U.S. Pat. No. 7,563,542, Jul. 21, 2009]. Otherhalogenated graphite materials, such as chlorinated graphite (C₈Cl) andbrominated graphite (C₈Br), can be obtained by making a graphitematerial and a halogen or halogen compound react at a temperaturegreater than room temperature. The presently invented process isapplicable to pristine versions and various chemically modified versionsof the above-listed graphitic materials.

The chemically functionalized nano graphene platelets produced with thisprocess typically comprise a significant portion of single-layergraphene. The chemical reaction can be controlled to occur only to anedge or edges of the nano graphene platelets or, alternatively, to anedge and at least one primary surface, graphene plane, of said nanographene platelets.

EXAMPLES Materials

Both pristine graphene and graphene oxide materials were obtained fromAngstron Materials, LLC (Dayton, Ohio) and the product codes were N002N(pristine) and N002P (graphene oxide). The multi-walled carbon nanotubes(MWNTs) were purchased from (purity >95%). The single-walled carbonnanotubes (SWNTs) were acquired from Shenzhen Nanotech Port Co. (purityof CNTs >90%, purity of SWNTs >50%). Succinic anhydride (98%), stannousoctoate ([CH₃(CH₂)₃ CH(C₂H₅)COO]₂Sn, 95%), and ethylene glycol (99%)were purchased from Aldrich and used as received. 2-Bromoisobutyrylbromide (98%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%),N,N-(dimethylamino)pyridine (DMAP, 98%), and boron trifluoride diethyletherate (BF₃.OEt₂, 98%) were purchased from Alfa Aesar and used asreceived. ε-Caprolactone (Acros, 99%) and palmitoyl chloride (Fluke,97%) were used without further purification. Sodium azide (95%),2-chloroethanol (99%), 3-chloropropylamine hydrochloride (98%),potassium tetrachloroplatinate(II) (K₂PtCl₄, 99%),N-methyl-2-pyrrolidinone (NMP), acetone, tetrahydrofuran (THF),N,N-dimethyl formamide (DMF), chloroform, and other solvents wereobtained from Shanghai Reagents Company and used as received. Glycidylmethacrylate (GMA, Alfa Aesar, 95%) was purified by passing through acolumn filled with basic alumina to remove the inhibitor. Triethylamine(Et₃N), dichloromethane (CH₂Cl₂), methyl methacrylate (MMA, Alfa Aesar,99%), and styrene (St, Alfa Aesar, 98%) were dried with CaH₂ anddistilled under reduced pressure before use. CuBr (Aldrich, 99.999%) wasobtained from Aldrich and purified according to the publishedprocedures.(19) The monomer of 3-azido-2-hydroxypropyl methacrylate(GMAN3) was prepared by reaction of GMA and sodium azide in the mixtureof water and THF (5/1 by volume) at room temperature for 48 h.

Large-Scale Synthesis of 2-Azidoethanol

In a typical procedure, a solution of sodium azide (195 g, 3.0 mol) indeionized water (780 mL) and 2-chloroethanol (120.8 g, 1.5 mol) was to a2000 mL three-neck round-bottom flask equipped with a condenser. Theflask was immersed in an oil bath at 70° C. and stirring was maintainedfor 96 h. After cooling to room temperature, the reaction mixture wasextracted with diethyl ether (5×100 mL). The extracts were dried overanhydrous MgSO₄ overnight, filtered, concentrated on a rotaryevaporator, and distilled under reduced pressure to produce an oil-like,colorless substance. The yield was 214.4 g or 82%.

Large-Scale Synthesis of 3-Azidopropan-1-amine

A solution of sodium azide (195 g, 3.0 mol) in deionized water (800 mL)was added into a three-neck round-bottom flask equipped with acondenser. Then 3-chloropropylamine hydrochloride (195 g, 1.5 mol)dissolved in 300 mL of deionized water was added. After continuedstirring at 75-78° C. for 96 h, the white precipitate (NaCl) was removedas a byproduct from the reaction mixture by filtration. The yellowfiltrate was basified with aqueous NaOH to pH≈10-11 and furtherextracted with diethyl ether (5×200 mL). The organic fraction was driedover anhydrous MgSO₄ overnight, filtered, concentrated on a rotaryevaporator, and distilled under reduced pressure to produce a colorlessoil. The yield was: 108.4 g, 72%. ¹H NMR (CDCl₃, δ, ppm): 3.35 (t, 2H,CH₂N₃), 2.78 (t, 2H, NH₂CH₂), 1.71 (p, 2H, CH₂CH₂CH₂), 1.27 (s, 2H,NH₂).

Synthesis of 4-(2-Azidoethoxy)-4-oxobutanoic Acid

Succinic anhydride (23.0 g, 0.230 mol) was added into a three-neckround-bottom flask equipped with a condenser and a dropping funnel.Under nitrogen atmosphere and magnetic stirring, freshly distilledmethylene chloride (150 mL), DMAP (2.3 g, 19 mmol) and freshly distilledEt₃N (46.46 g, 0.460 mol) was sequentially added. After the flask wasimmersed into an ice—water bath, 2-azidoethanol (20.0 g, 0.230 mol) wasadded dropwise into the previous solution. The solution was later heatedat 40° C. for 48 h, and the reaction mixture was washed successivelywith 1 M HCl solution (5×100 mL) and deionized water (2×100 mL). Theorganic phase was dried over anhydrous MgSO₄ overnight. After filteringand removal of methylene chloride under reduced pressure, the finalproduct was obtained as a yellow viscous liquid. The yield was: 38.4 g,90%. ¹H NMR (CDCl₃, δ, ppm): 4.20 (t, 2H, N₃CH₂CH₂), 3.42 (t, 2H,N₃CH₂), and 2.61 (m, 4H, CH₂CH₂COOH).

Synthesis of 2-Azidoethyl-2-bromo-2-methylpropanoate

2-Azidoethanol (17.40 g, 0.2 mol), freshly distilled methylene chloride(150 mL), and Et3N (21.21 g, 0.21 mol) were added into a three-neckround-bottom flask equipped with a condenser and a dropping funnel.Under nitrogen atmosphere and magnetic stirring, freshly distilledanhydrous methylene chloride (150 mL), DMAP (1.7 g, 14 mmol), andfreshly distilled anhydrous Et3N (46.5 g, 0.46 mol) were sequentiallyadded. After the flask was immersed into an ice—water bath,2-bromoisobutyryl bromide (48.28 g, 0.21 mol) was added dropwise intothe previous solution. Twenty-four hours later, the reaction mixture waswashed successively with 1 M HCl (3×200 mL) solution and deionized water(1×200 mL). The organic phase was dried over anhydrous MgSO₄ overnight.After filter and removal of methylene chloride on a rotary evaporator,the obtained residues were distilled under reduced pressure to give acolorless viscous liquid. The yield was: 33.4 g, 70%. ¹H NMR (CDCl₃, δ,ppm): 4.24 (t, 2H, N₃CH₂CH₂), 3.52 (t, 2H, N₃CH₂), 1.96 (s, 6H,(CH₃)₂Br).

Preparation of NGP-OH, NGP-NH₂, NGP-COOH, and NGP-Br

In a typical experiment (feed ratio, R_(feed)=20/1 (w/w)), pristine NGPs(1.00 g) and N-methyl-2-pyrrolidinone (NMP, 80 mL) were placed in a 250mL Schlenk flask fitted with a condenser. The mixture was treated withan ultrasonic bath (40 kHz) for 2 h and then placed on a magneticstirrer with an oil bath. After the mixture was bubbled with nitrogenfor 15 min, 2-azidoethanol (20.0 g, 0.23 mol) was added via syringe. Thereaction mixture was then heated and maintained around 160° C. in anitrogen atmosphere under constant stirring for 18 h. After cooling downto room temperature, the product was isolated by precipitation intoacetone. The resulting precipitates were re-dispersed in acetone withthe aid of an ultrasonic bath and then collected by centrifugation. Thiscentrifugation was repeated until the upper layer was nearly colorless.The separated solid was sequentially re-dispersed in water and purifiedby at least five centrifugation cycles. All these centrifugations wereperformed at a rotation speed of at 14,500 rpm for 3 min using 30 mLplastic centrifuge tubes. The supernatant was decanted and the blacksolid was dried under vacuum at 60° C. overnight to give 1.04 g ofNGP-OH. Thus, this is a weight-increase process, and the mass loss ofneat NGPs during the preparation methods is less than 10%. Oneadditional batch of 5-10 g of non-pristine NGPs (GO) and similar,dispersed functional NGPs were obtained.

The same procedure was also used to prepare NGP-NH₂, NGP-COOH, andNGP-Br, but 2-azidoethanol was substituted by 3-azidopropan-1-amine,4-(2-azidoethoxy)-4-oxobutanoic acid, and 2-azidoethyl2-bromo-2-methylpropanoate, respectively.

Synthesis of NGP-g-PCL by Ring-Opening Polymerization (ROP)

Into a 50 mL Schlenk flask, as-prepared NGP-OH(R_(feed)=20/1, 50 mg) wascharged, and the flask was then sealed with a rubber plug. The flask wasevacuated and filled thrice with high-purity nitrogen. ε-Caprolactone(6.0 g, 53 mmol) and stannous octoate (2 mg) were injected into theflask via a syringe. The reaction was allowed to proceed for 24 h at120° C. under constant stirring. The product was filtered and washedthoroughly with excess chloroform several times. The final product wasdried under vacuum overnight to give 54 mg of NGP-g-PCL.

Synthesis of NGP-g-PGMA by Cationic ROP

Into a dried Schlenk flask as-prepared MWNT-OH (Rfeed=20/1, 20 mg),dried CH₂Cl₂ (15 mL) and GMA (4.0 g, 28 mmol) were added under nitrogen.The flask was then treated with ultrasonic bath for 1 min before placedinto ice—water bath. BF₃.OEt₂ (0.1 mL) was injected into the reactionmixture by syringe quickly. After 24 h, the cationic polymerization wasended by adding a small amount of methanol. The resulting product waswashed with methanol and separated by centrifuging. The final productwas dried under vacuum overnight to give 17 mg of NGP-g-PGMA.

Synthesis of NGP-PC by Amidation

NGP-NH₂ (20 mg) was dispersed via ultrasonication in 8 mL of driedCHCl₃. After dried Et3N (3.5 g, 35 mmol) and palmitoyl chloride (3 g, 11mmol) were added, the reaction was allowed to proceed at roomtemperature for 24 h. The product was isolated by centrifugation andrinsed in turn with 1 M HCl, deionized water, and acetone. The blacksolid was collected and dried overnight under vacuum to give 18 mg ofNGP-PC.

Preparation of Pt/NGP Nano-Hybrids

The as-prepared NGP-COOH (20 mg) and 40 mL of ethylene glycol—watersolution (3:2 volume ratio) were placed into a 100 mL Schlenk flask,which was then treated with an ultrasonic bath (40 kHz) for 3 min.K₂PtCl₄ (12.8 mg, 0.03 mmol) was added into the flask before thereactive mixture was heated in a 125° C. oil bath under nitrogenatmosphere for 4 h. The product was centrifuged, rinsed several timeswith deionized water, and dried at 60° C.

Synthesis of NGP-g-PMMA by Atom Transfer Radical Polymerization (ATRP)

To a 25 mL Schlenk flask containing a magnetic stirrer, 50 mg of NGP-Brwas dispersed in 2 mL of THF upon sonication for 15 min before MMA (0.5g, 5 mmol), CuBr (11.5 mg, 0:08 mmol), and PMDETA (17 μL, 0.08 mmol)were added under nitrogen. The flask was then sealed and stirred at 40°C. for 24 h. The mixture showed obvious viscosity at the end of thereaction. The mixture was subsequently diluted to THF, centrifuged, andrinsed several times with THF to remove any un-grafted polymer. A blacksolid NGP-g-PMMA (66 mg) was obtained after vacuum-drying overnight.

Synthesis of NGP-g-PS by ATRP

In a 25 mL Schlenk flask, 50 mg of MWNT-Br was dispersed via sonicationfor 15 min in 0.91 g of styrene. CuBr (18.5 mg, 0.13 mmol) and PMDETA(21 μL, 0.10 mmol) were added under a nitrogen atmosphere. The flask wasplaced in an oil bath at 80° C. under magnetic stirring. After 24 h, theviscosity was clearly increased. The mixture was cooled to roomtemperature and washed by repeated dispersing in THF and centrifuging.The black solid was collected and dried under vacuum at 30° C. to aconstant weight, giving rise to 24 mg of NGP-g-PS.

Synthesis of NGP-g-GMAN3 by ATRP

In a 25 mL Schlenk flask, 30 mg of NGP-Br was dispersed via sonicationfor 15 min in 1.5 mL of THF. GMAN3 (0.40 g, 2.2 mmol), CuBr (7.8 mg,0.05 mmol) and PMDETA (11 μL, 0.05 mmol) was added under a nitrogenatmosphere. The resulting mixture was stirred for 24 h at 25° C. Thesolid was then separated from the mixture by centrifuging and washedwith acetone. The black solid was collected and dried under vacuum at30° C. to a constant weight, providing 16 mg of NGP-g-GMAN3.

Characterization of Materials Synthesized:

A combination of the following techniques, when deemed necessary, wasused to characterize the functional groups attached to NGPs forfunctionalized versions of both pristine graphene and graphene oxide:

-   -   (1) Thermogravimetric analysis (TGA) was used to determine the        level of surface functionalization. Since most functional groups        were labile or decompose upon heating, and the NGPs are stable        up to 1200° C. under argon (Ar) atmosphere, the weight loss at        800° C. under Ar was used to determine functionalization ratio.    -   (2) X-ray photoelectron spectroscopy (XPS) was used to confirm        the presence of different elements in functionalized NGPs.        De-convolution of XPS signals was useful for studying fine        structures on NGPs.    -   (3) Raman spectroscopy: The tangential G mode (ca. 1550-1600        cm⁻¹) was characteristic of sp² carbons on the hexagonal        graphene network. The D-band, so-called disorder mode (found at        ca. 1295 cm⁻¹), appears due to disruption of the hexagonal sp²        network of NGPs. The D-band was be used to characterize        functionalized NGPs and ensure that functionalization is        covalent and occurs on the primary surfaces of a graphene sheet.    -   (4) Infrared (IR) spectroscopy was useful in characterizing        functional groups bound to graphene surfaces. A variety of        organic functional groups on graphene surfaces, such as COOH(R),        —CH₂, —CH₃, —NH₂, and —OH, were identified using FTIR.    -   (5) UV/visible spectroscopy was used to provide information        about the electronic states of NGPs, and hence        functionalization. The absorption spectra showed bands near 1400        nm and 1800 nm for pristine NGPs. A loss or shift of such        structure was be observed after chemical alteration of NGP        surfaces.    -   (6) Solution ¹H NMR was of adequate sensitivity for        characterizing NGPs functionalized by carbenes and nitrenes        because of the high solubility of derivatized NGPs. Solid state        ¹³C NMR was employed to characterize several functionalized NGPs        and show successful observation of organic functional groups,        such as carboxylic and alkyl groups on graphene surfaces and        edges.    -   (7) Atomic force microscopy (AFM) and transmission electron        microscopy (TEM were used to characterize both un-treated and        functionalized NGPs. The height profile on AFM was used to show        presence of functional groups on a NGP surface. Measurements of        heights along an individual graphene plane could be correlated        with the substituent group, i.e., the larger an alkyl chain of a        surface substituent, the greater the height measured.

1. A combined production-functionalization process for manufacturing achemically functionalized nano graphene material directly from apristine graphite material, comprising: (A) Dispersing said pristinegraphite material and an azide or bi-radical compound in a liquid mediumto form a suspension, wherein said pristine graphite material isselected from the group consisting of natural graphite, artificialgraphite, highly oriented pyrolytic graphite, carbon fiber, graphitefiber, carbon nano-fiber, graphitic nano-fiber, meso-carbon micro-bead,graphitized coke, and combinations thereof; (B) Subjecting saidsuspension to direct ultrasonication with ultrasonic waves of a desiredintensity for a length of time sufficient to produce nano grapheneplatelets and to enable a chemical reaction to occur between said nanographene platelets and said azide or bi-radical compound to produce saidfunctionalized nano graphene material.
 2. The process of claim 1 whereinsaid azide or bi-radical compound is added to said liquid mediumsequentially after said direct ultrasonication of said graphite materialis allowed to proceed for a first period of time.
 3. The process ofclaim 1 wherein said nano graphene platelets comprise single-layergraphene.
 4. The process of claim 1 wherein said chemical reactionoccurs only to an edge or edges of said nano graphene platelets.
 5. Theprocess of claim 1 wherein said chemical reaction occurs to an edge andat least one primary surface, graphene plane, of said nano grapheneplatelets.
 6. The process of claim 1 wherein said azide or bi-radicalcompound is selected from the group consisting of 2-Azidoethanol,3-Azidopropan-1-amine, 4-(2-Azidoethoxy)-4-oxobutanoic acid,2-Azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R—)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.
 7. The process of claim 1, further comprisinga step of grafting a polymer chain to a functional group of saidfunctionalized nano graphene material to produce a polymer-grafted nanographene material.